Difference between '+' and concat() method in String

[Vijaylaxmi Agarwal]

Hi,

Can anybody tell me what is the difference between '+' and concat() method in String.

Thanks in advance

Vijaylaxmi

[arulk pillai]

I think there is no difference. I think both use a StringBuffer under the covers and do a toString() method at the end.

[Rob Spoor]

Not quite. + does use a StringBuffer or StringBuilder, but concat() creates a new char[] with the contents of the current String and the String to append, and returns a new String based on that array:

public String concat(String str) {
    int otherLen = str.length();
    if (otherLen == 0) {
        return this;
    }
    char buf[] = new char[count + otherLen];
    getChars(0, count, buf, 0);
    str.getChars(0, otherLen, buf, count);
    return new String(0, count + otherLen, buf);
}

I can actually see one more improvement on that which Sun have overlooked:

if (count == 0) {
    return str;
}

Now, if you concatenate only 2 strings the difference in performance will be minimal, but otherwise using + will result in fewer objects created, if there is a difference. However, its internal capacity can increase exponentially (it tries to use (value.length + 1) * 2 if the contents would not fit), so in the end it could be worse.

Still, using + is the usual way to go. Don't start using concat unless you find you really need to improve on performance.